The present invention relates to a bonding type semiconductor substrate and a semiconductor light emitting element, and preparation processes thereof. Particularly, the present invention relates to a bonding type semiconductor substrate based on a novel wafer direct bonding technique, a light emitting element such as LEDs (light emitting diode) with a high brightness obtained by applying the technique, and preparation processes thereof.
A light emitting element based on a conventional technique will be explained with reference to drawings attached hereto. FIG. 22 shows an embodiment of a visible light LED of InGaAlP based on a conventional technique.
In an LED 100 shown in FIG. 22, InGaAlP epitaxial growth layers 84, 85, 86 contributing to light emission are formed on an n-type GaAs substrate 82. Although not shown in this drawing, a buffer layer may be disposed between the substrate and the epitaxial growth layer in compliance with required specifications in order to obtain an excellent light emitting layer.
Electrodes 89 for supplying an electric current are each disposed on the upper surface of the epitaxial growth layer 86 and the lower surface of the substrate 82, respectively. Although not shown in the same drawing, a layer for diffusing the electric current or a layer for taking an electric contact is often disposed between an upper electrode 89 and the epitaxial growth layer 86. Among the epitaxial growth layers 84, 85, 86, the layer which can emit light by the recombination of carriers is the active layer 85. The epitaxial layers 84 and 85 formed on and under the active layer 85 are the cladding layers 84 and 86 having a wider band gap than the active layer in order to confine the carriers and to thereby heighten an emission efficiency.
For these epitaxial growth layers 84, 85 and 86, the band gap is required to be optimized according to a design for the purpose of adjusting the wavelength of the emission and for the purpose of confining the carriers. Furthermore, it is desirable for a good epitaxial growth that a lattice constant of the epitaxial growth layer matches a lattice constant of the substrate 82. Since InGaAlP which is a group compound contains three elements of In, Ga and Al as components in the group III, the band gap and the lattice constant can be independently designed by selecting a composition ratio of these elements.
For example, when a composition of the epitaxial growth layer is represented by the following formulaInx(Ga1-yAly)1-xP   (1),the lattice constant of the epitaxial growth layer almost matches that of a GaAs substrate by setting a composition ratio (x) of In to 0.5. The band gap can be controlled by adjusting a composition ratio (y) between Al and Ga, while x=0.5 is kept up.
For example, in order to obtain a red light emitting LED having a wavelength of 644 nm, the composition ratio of the active layer 85 is set to x=0.5 and y=0.043, and the composition ratio of the cladding layers 84, 86 is set to x=0.5 and y=0.7. Moreover, in order to obtain a green light emitting LED having a wavelength of 562 nm, the composition ratio of the active layer 85 is set to x=0.5 and y=0.454, and the composition ratio of the cladding layers 84, 86 is set to x=0.5 and y=1.00, i.e., InAlP.
As illustrated in the above, for InGaAlP epitaxial growth layers, the wavelength of the emission can be selected within a region of visible light. Furthermore, since the layers can epitaxially grow under the lattice matching condition with a GaAs substrate which is most general as a compound semiconductor substrate, there exist advantages that the substrate is easily available and the epitaxial growth is relatively easy.
On the contrary, a GaAs substrate has a disadvantage that it absorbs light in a region of visible light. Therefore, part of the light emitted on an InGaAlP epitaxial growth layer is absorbed by GaAs substrate, and hence a brightness of LED decreases unavoidably. For avoiding the decrease of the brightness, a material which is transparent in the region of the visible light is preferably employed as a substrate. The usual transparent material is GaP, but since the GaP substrate cannot obtain the lattice matching with InGaAlP, the good epitaxial growth is difficult. In order to solve this problem, U.S. Pat. No. 5,376,580 filed in 1993 proposes a method for a wafer bonding between an InGaAlP epitaxial growth layer and a GaP substrate. This proposed method comprises removing a GaAs substrate from the epitaxial growth layer, closely bonding a GaP substrate instead of the GaAs substrate, and then carrying out a heat treatment under pressure to integrally bond them. According to this method, the increase of the brightness of LED can be attained. However, the stability and productivity of the wafer bonding step are insufficient, because the epitaxial growth layer after the removal of the GaAs substrate is thin and thus its handling is difficult, and a special apparatus is necessary for the heat treatment under pressure.
Next, the following explains the wafer bonding. If two different kinds of wafers can be integrally bonded, a laminated structure comprising different materials could be obtained regardless of the lattice constant, and a different substance can be buried inside as represented by SOI (silicon on insulator). For these reasons, various wafer bonding techniques have been proposed hitherto. For example, the above-mentioned bonding method of subjecting two wafers to the heat treatment while they are pressed is disclosed in Japanese Patent No. 765892 filed in 1970. The wafer bonding technique has been desired for a long time, but it is difficult to accomplish the integral bonding all over the surface of the wafer, so that this technique has not been practiced.
The present inventors have developed a technique called “direct bonding” or “direct joining” as a practicable technique. For example, Japanese Patent No. 1420109 filed in 1983 and the like describes the direct bonding between Si wafers, and Japanese Patent No. 2040637 filed in 1985 and the like describes the direct bonding between compound semiconductor wafers.
The direct bonding technique comprises mutually closely bonding two substrates having mirror-finished surfaces by themselves at room temperature under a substantially dust-free atmosphere, and then integrally joining them by a heat treatment. This technique has an advantage that the whole surface can be joined without leaving any unbonded part, because the whole surfaces are closely bonded to each other prior to the heat treatment. Moreover, it is not necessary to apply a pressure during the heat treatment, any special apparatus or equipment is not required. The mechanism of the direct bonding between the Si wafers is understood as follows.
Namely, at the beginning, OH groups are formed on the surface of the wafer by cleaning or washing with water. Then, when the surfaces of the two wafers are contacted with each other, the OH groups attract each other by a hydrogen bond, so that the wafers closely bond at room temperature. A bonding power in this case is strong enough to eliminate a usual warp of the wafer, whereby the close bonding all over the surfaces can be achieved. During the heat treatment, a dehydrative condensation (Si—OH:HO—Si→Si—O—Si+H2O) occurs at a temperature higher than 100° C., and the wafers bond to each other via oxygen atoms, thereby increasing a bonding strength. When the temperature further rises, the diffusion and rearrangement of the atoms in the vicinity of a bonding interface occur, so that the wafers are integrated mechanically and electrically. The bonding mechanism of the compound semiconductors is considered to be similar.
Next, one example of a preparation process of an LED comprising an InGaAlP-based epitaxial growth layer closely bonded to the GaP substrate by utilizing the direct bonding will be explained with reference to FIG. 23.
First, as shown in FIG. 23A, an n-type cladding layer 94, an active layer 95 and a p-type cladding layer 96 are grown on an n-type GaAs substrate 92. Then, as shown in FIG. 23B, a GaP substrate 91 is directly bonded to the surface of the epitaxial growth layer 96. Furthermore, shown in FIG. 23C, the GaAs substrate 92 is removed by polishing or etching, and after upside down, electrodes 99 are disposed on the upper surface of the n-type cladding layer 94 and under the lower surface of GaP substrate 91, respectively, so that an InGaAlP-based LED having GaP as the substrate 91 is obtained as shown in FIG. 23D.
In the case that the different materials are directly bonded to each other in such a manner, especially in the case that the surfaces of the epitaxial growth layers are directly bonded, the following problems exist, in contrast to the direct bonding of the same wafers such as Si wafers or GaAs wafers.
First, on the surface of the epitaxial growth layer, particles such as dust are more easily deposited than on the surface of the wafer. Accordingly, the bonding at room temperature is hindered, and even after the heat treatment, the boding is not accomplished all over the surface and hence there is a problem that unbonded portion called “voids” occur. Generally, the surface of a wafer is kept clean and a clean substrate is employed as a substrate for epitaxial growth. However, particle adhesion on the surface of the epitaxial growth layer is inevitable to some extent at present, since reaction products are deposited during the epitaxial growth and dust particles adhere in a pre-treatment step or a post-treatment step of the epitaxial growth step.
Second, since the wafer is warped by the epitaxial growth, there is a problem that it is impossible to bond the whole surface of the wafers closely at room temperature.
Third, since difference of thermal expansion exists between different materials, there is a problem that thermal stress occurs during the heat treatment, and thereby the bonded substrate is broken by the stress.
Fourth, since difference of thermal expansion exists between different Materials, a gap at the bonding interface occurs during the heat treatment for bonding even if the breakage of the bonded substrate does not occur, and thus the whole surface of the substrate cannot be bonded homogeneously by “the gap”.
Fifth, there is a problem that electric resistance is generated at the bonding interface. The original investigation of the present inventors discloses that an electrically resistive component is generated by bonding the wafers. When LED, for example, is formed using such a bonded substrate, the electric resistance at the bonding interface increases operating voltage of LED and thus problems such as a poor emission and a heat generation occur.